In classical thermodynamics the most efficient closed cycle heat engine is known as the "Carnot engine" operating on the reversible "Carnot cycle". If T.sub.h and T.sub.l denote the temperatures of the high and low temperature heat reservoirs respectively of a Carnot engine, the theoretical output work W is given by ##EQU1## where Q denotes the input thermal energy taken from the high temperature heat reservoir. The most efficient cooling system (i.e., refrigerator) is known as a "Carnot refrigerator". It is simply a Carnot engine operating in reverse. In this case Q, in the above equation, represents the amount of heat taken from the low temperature reservoir and transferred to the high temperature reservoir, and W represents the amount of input work required to achieve the transfer. For refrigerators, t.sub.l and t.sub.h are reversed in the above equation.
The natural environment at ambient temperature plays a key role in cyclic heat engines and refrigerators that operate by subjecting their working fluids to purely thermodynamic processes within the theoretical framework of thermodynamics. It represents a temperature zone which divides the operating temperature regimes of cyclic heat engines and refrigerators. This is because the environment at ambient temperature represents the low temperature heat reservoir for cyclic heat engines which operate by absorbing heat energy from a high temperature reservoir above ambient temperature and generating mechanical work, while in refrigerators it represents the high temperature heat reservoir which operate by absorbing heat energy from a low temperature reservoir below ambient temperature and consuming mechanical work.
The reason why closed cycle condensing heat engines are forced to operate above ambient temperature is because according to the principles of thermodynamics there is only one possible method for reducing the entropy of the working fluid required for a condensing system so that the engine can be operated cyclically. This method involves extracting heat energy from the working fluid inside the condenser and transferring it to a heat sink that is at a lower temperature. The natural environment at ambinet temperature is utilized as this heat sink and represents the low temperature heat reservoir. Since it is impossible to reduce the entropy of a working fluid without the usual method of heat transfer to a heat sink by thermodynamic processes, all prior art closed-cycle condensing heat engines operating under purely thermodynamic principles and processes must operate above ambient temperature.
There is one type of heat engine that can be operated below ambient temperature that is capable of producing both mechanical work and refrigeration. This engine is a "cryogenic engine". In this engine liquefied working fluid at cryogenic temperature (such as liquefied nitrogen at 77.degree. K. which is the usual working fluid in cryogenic engines) is compressed to very high pressure (e.g., 300 Bar) by a hydraulic compressor and fed through a plurality of serially connected heat exchangers maintained in thermal contact with the natural environment at ambient temperature, and a like plurality of expanders interposed between adjacent heat exchangers. The high pressure liquefied working fluid entering the first heat exchanger creates a significant temperature gradient across the thermal surfaces and a large amount of natural heat energy is extracted from the environment at ambient temperature and rapidly absorbed by the circulating working fluid at cryogenic temperature. This produces a strong refrigeration effect. The liquefied working fluid is isobarically heated above its critical temperature (126.3.degree. K. in the case of nitrogen working fluid) and completely vaporized into a super high pressure gas.
The cryogenic working fluid emerges from the first heat exchanger as a super high pressure, superheated gas at about ambient temperature. It is then fed into the first isentropic expander where heat energy taken from the natural environment in the first heat exchanger is converted into mechanical work. The pressure ratio of the first expander is such that the outlet pressure of the expanded gas leaving the expander is still fairly high. Thus, since the expansion process reduces the temperature of the exhaust gas significantly below ambient temperature, it is fed into another ambinet heat exchanger that is also maintained in thermal contact with the natural environment in order to extract still more natural thermal energy thereby providing additional refrigeration. After this second isobaric heating process, the pressurized gas is withdrawn from the second ambient heat exchanger at about ambient temperature and fed into a second isentropic expander where natural thermal energy extracted from the environment while circulating through the second heat exchanger is converted into additional mechanical work. This process of absorbing natural thermal energy from the environment and converting it into mechanical work while simultaneously providing refrigeration is continued until the exhaust pressure of the gas emerging from the last expander is equal to atmospheric pressure whereupon the gas is discharged into the open atmosphere. The operating details of this cryogenic engine can be found in U.S. Pat. No. 3,451,342 filed Oct. 24, 1965 by E. H. Schwartzman entitled "Cryogenic Engine Systems and Method".
Although this heat engine operates below ambient temperature of the natural environment and generates both mechanical work and refrigeration, it is not a cyclic heat engine. When the supply of liquefied working fluid at cryogenic temperature is consumed, the engine (and refrigerator) stops operating. Since the engine operates by strictly thermodynamic processes according to the principles of thermodynamics, the expanded working fluid cannot be recondensed into a liquid at cryogenic temperature because there is no natural heat sink available at cryogenic temperature to absorb heat energy. Thus, there is no thermodynamic method that can be used to reduce its entropy in order to enable the engine to operate cyclically. However, there is a non-thermodynamic method that can be used to reduce the entropy of the working fluid of a heat engine without having to transfer heat energy to a heat sink if the working fluid is paramagnetic. This method represents the underlying operating principle of the present invention disclosed herein.
It follows from the Carnot equation for refrigerators that when T.sub.l .fwdarw.0, the required input work W.fwdarw..infin.. Thus, it is a physical impossibility to achieve temperatures below approximately 0.4.degree. K. by using strictly thermodynamic processes. For many years this temperature (0.4.degree. K.) was believed to represent a "temperature barrier" which could not be broken because of basic laws of thermodynamics. However, in 1926 Debye proposed using an electromagnetic process that is outside the theoretical framework of classical thermodynamics (i.e., that is not a thermodynamic process) to break this thermodynamic barrier and achieve temperatures that are many orders of magnitude below 0.4.degree. K. This process is called "adiabatic demagnetization" or "magnetic cooling". Basically, this process involves subjecting a paramagnetic material at low temperature (usually a solid paramagnetic salt) to a very intense magnetic field thereby heating the material while the entropy remains constant. When the heat of magnetization is extracted by a cryogenic heat sink (e.g., liquid helium at 1.degree. K.) the entropy of the magnetized material decreases. By thermally isolating the material and removing the magnetic field, the entropy of the material remains constant but the temperature will fall way below that of the heat sink. By using this non-thermodynamic electromagnetic process (known as the "magneto-caloric effect"), temperatures as low as 0.0001.degree. K. are possible.
It is important to point out and emphasize that when electromagnetic processes, such as the magneto-caloric effect, are used in conjunction with thermodynamic processes, the results can no longer be predicted within the theoretical framework of classical thermodynamics. For example, when subjecting a paramagnetic substance to a magnetic field, the temperature of the substance increases but its entropy (i.e., the degree of random molecular motion) remains constant due to magnetic alignment. This is thermodynamically impossible. According to thermodynamics, a substance that is heated always results in an increase in entropy. This illustrates the fact that thermodynamic principles cannot be applied to non-thermodynamic processes. (See, "Classical Physics Gives Neither Diamagnetism nor Paramagnetism," Section 34-6, page 34-8, in The Feynman Lectures On Physics, by R. Feynman, Addison-Wesley Pub. Co., 1964.)
The object of the present invention is to utilize the magneto-caloric effect to provide a condensing system that does not require a low temperature heat sink. Such a system could be used to construct closed-cycle condensing cryogenic engines that could be used to produce both mechanical work and refrigeration.
A recent technical development that is exploited in the design of the condensing system disclosed herein is the discovery of superconducting materials with critical temperatures above the boiling temperature of liquid nitrogen See the article, "Superconductivity Seen Above The Boiling Point of Nitrogen," Physics Today, April 1987, pp. 17-23 by Anil Khurana. Since cryogenic engines use these fluids (liquefield nitrogen, etc.,) at cryogenic temperature in their basic operation, this development means that it is now possible to utilize the working fluids of cryogenic engines as a cryogenic coolant for superconducting magnets instead of liquid helium which is very expensive. Since superconducting magnets generate intense magnetic fields without consuming any energy, it is possible to utilize these intense magnetic fields to construct a condensing system without requiring any external refrigeration system for the superconducting magnet. The reason why this is possible is because ordinary oxygen gas, which can be used as a working fluid in cryogenic engines, is highly paramagnetic. Since there is no cryogenic heat sink available, condensation can only be achieved by isentropically expanding low temperature vapor inside a thermally insulated condensing chamber maintained at very low pressure. However, since only a portion of the vapor can be condensed by this expansion process (via spontaneous condensation of supersaturated vapor), it is necessary to continuously remove the noncondensed portion in order to maintain the required vacuum environment inside the condensing chamber so that the condensing process can continue. By utilizing oxygen as the engine's cryogenic working fluid, this can be achieved magnetically while expending relatively little mechanical work. The high entropy noncondensed oxygen vapor can be continuously removed from the low pressure condensing chamber by means of magnetic forces generated by a superconducting solenoid. The vapor is pulled out of the chamber into the bore of the solenoid, magnetized, and magnetically compressed. Since the vapor is at cryogenic temperature, it is possible to approach paramagnetic saturation by employing a sufficiently strong magnetic field.
The magnetic forces accelerate the gas molecules moving into the magnetic field thereby increasing their kinetic energy. This increase in kinetic energy represents the heat of magnetization. By mounting a low pressure, non-magnetic rotating turbine in the accelerating gas stream, this directed magnetic kinetic energy can be extracted from the molecules, transferred to the rotating turbine and converted into mechanical work with nearly 100% conversion efficiency. The gas molecules arrive at the most intense region of the magnetic field without any significant increase in kinetic energy. Thus, the process is essentially equivalent to isothermal magnetization. A large percentage of the gas molecules will have their magnetic dipole moments aligned with the external field which results in a decrease in the entropy of the vapor. This magnetically compressed low entropy vapor is further compressed by a non-magnetic turborecompressor mounted inside the bore of the solenoid such that the vapor is forced out of the solenoid, demagnetized to a thermodynamic state identical to the preexpansion state, mixed with previously condensed vapor, and recycled back through the condensing expander to continue the condensing process. Since the entropy of the noncondensed vapor inside the solenoid is lower than it would ordinarily be without the magnetic field, the mechanical work consumed by the recompressor is reduced. Thus, the mechanical work required to maintain the vacuum environment of the condensing system is reduced. The liquefield oxygen withdrawn from the condensing system can be used to maintain the cryogenic temperature of the superconducting solenoid and utilized as the working fluid for a cyclic cryogenic engine. These are the basic physical principles and operating features of the invention disclosed herein.